Issue 30

S. Baragetti et alii, Frattura ed Integrità Strutturale, 30 (2014) 84-94; DOI: 10.3221/IGF-ESIS.30.12

Quasi-static SCC data The results of the quasi-static testing campaign in air and methanol at different concentrations are reported in Fig. 6.

Figure 6 : Quasi-static experimental data and interpolations with confidence bands for Ti-6Al-4V specimens tested in air and methanol water solution at different concentrations, in terms of maximum nominal stress in the test section. The quasi-static results indicate that the SCC behavior show a reduction of the failure stress starting from a 90% wt. methanol concentration in water. The overall drop is limited for pure methanol, with a -25% in terms of nominal stress applied to the test section. However a marked methanol effect is recognizable even for a 92.5 % methanol concentration, if compared to higher results from literature [10]. Comparison between Corrosion Fatigue and Quasi-static SCC data The results in terms of maximum stress drop in function of methanol concentration have been extracted from the experimental data presented in this work, and compared between the fatigue and quasi-static tests. The chemical driving forces in the two different cases have been highlighted by extracting the difference from the mechanical failure stress, according to the following relations:     , , , , chem QS mech QS QS met chem F mech F F met X X           The direct comparison is presented in Fig. 7. Fig. 7 remarks the difference in terms of the material strength behavior in an aggressive methanolic media for the Ti-6Al- 4V alloy for different loading conditions. It must be recognized that a dynamic loading causes an onset of corrosive effect even from mild methanol concentrations, while a methanol concentration over 90% is needed in a quasi-static SCC condition. The different loading conditions may affect the protective role of the surface oxide of the alloy showed in [3,4,6]. In particular, fatigue loading may induce premature cracking of the substrate oxide, resulting in an enhanced susceptibility of the material to the external environment, while quasi-static conditions require higher methanol concentrations to damage the external film. Further analyses at different loading conditions, number of cycles and frequencies are needed in order to evaluate the mechanical loading effects on the corrosion protection characteristics of the Ti-6Al-4V alloy.

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